Theoretical and Experimental Investigation of the Spatial Filtering Properties of a Number of Concentric Annular Strain Sensors

As an extension to the work done on shaped sensors as spatial filters in one dimension, spatial filtering in two dimensions is considered. The spatial windowing function of disc shaped and annular strain sensors are derived. Experimental results are shown to validate the model. A numerical method for combining annular sensors is then presented. It is shown that low and band pass sensors can be created with relatively few annular elements if the evanescent field is not significant at the sensor. Theoretical and experimental results are presented as well as design criteria. Limitations due to spatial sampling, anisotropic properties of strain sensitive materials and the effect of evanescent and in-plane structural waves are discussed.